1. Field of the Invention
The present invention relates to a non-aqueous pigment ink, and more particularly to a non-aqueous pigment ink that is suited to use within an inkjet recording system.
2. Description of the Related Art
Inkjet recording systems are printing systems in which printing is conducted by spraying a liquid ink with a high level of fluidity from ultra-fine nozzles, and adhering that ink to a recording medium such as a sheet of paper. These systems are comparatively cheap and offer the advantage of enabling high-resolution, high-quality images to be printed rapidly and with minimal noise, and consequently continue to spread rapidly.
The coloring materials for the inks used in these inkjet recording systems can be broadly classified into materials that use pigments and materials that use dyes. Of these, there is a growing tendency for the use of inks that use pigments as the coloring materials, as such inks exhibit the excellent levels of light resistance, weather resistance and water resistance that are required for high image quality printing.
In terms of the solvent, inks can be broadly classified into aqueous inks and non-aqueous inks. In an aqueous ink, because an aqueous solvent and water act as the ink medium, finely dispersing a pigment within this medium and then maintaining the stability of that dispersion is extremely difficult.
As a result, aqueous pigment inks have been proposed in which the pigment is encapsulated, thereby enabling dispersion within an aqueous medium (see Japanese Patent Laid-Open No. H09-151342 and Japanese Patent Laid-Open No. H11-140343). However, because the inks are water-based, the problem of poor water resistance is unavoidable.
In contrast, non-aqueous inks that do not use water for the ink solvent, including solvent-based inks that use a volatile solvent as the main constituent and oil-based inks that use a non-volatile solvent as the main constituent, are now attracting considerable attention. Non-aqueous inks exhibit superior drying properties to aqueous inks, and also offer excellent printability.
These non-aqueous inks typically comprise a non-aqueous solvent, a binder resin, and a pigment and the like. One problem associated with inkjet pigment inks is the problem of ink blockages within the printing head. Methods that have been proposed to overcome this problem include increasing the quantity of solvent, and adjusting the viscosity of the ink to a viscosity appropriate for inkjet spraying.
However, increasing the quantity of solvent causes a reduction in the resin concentration, and because the resin penetrates into the recording medium together with the solvent, the binder strength becomes inadequate and the pigment fixation tends to deteriorate. These tendencies are particularly marked in those cases where the recording medium is a readily permeable paper such as plain paper.
On the other hand, if the resin quantity is increased in order to improve the fixation of the pigment to the recording medium and the scratch resistance, then the viscosity of the ink increases, which increases the likelihood of nozzle blockages.
Accordingly, current inks are prepared with either the ink viscosity or the pigment fixation given priority.
Moreover, compared with aqueous inks, non-aqueous inks exhibit a higher level of affinity between the coloring material and the solvent, and consequently a problem arises in that when the ink bonds to the paper, the coloring material does not remain at the paper surface, but rather penetrates into the interior of the paper with the solvent, causing a reduction in the ink density at the printed surface (the upper surface) of the printed item, and increasing the likelihood of strike-through at the non-printed surface (the underside). Furthermore, in the case of pigment inks, a pigment dispersant is usually added to improve the dispersibility of the pigment, but increasing the blend quantity of the dispersant in order to ensure favorable storage stability tends to cause an associated deterioration in the scratch resistance.